The present invention relates to lasers and, in particular, to windows for lasers having improved transmission qualities.
A typical gas laser, such as an ion or krypton ion laser has a discharge tube or envelope enclosing the gaseous laser medium. A discharge is established through the lasing medium to excite the lasing medium to elevated energy states required for lasing action. An optical resonator or cavity is aligned with the discharge tube so that light emitted by the excited lasing medium oscillates between the optical resonator mirrors and is amplified as it passes through the lasing medium itself.
The ends of the discharge tube typically are terminated by highly light transmissive windows. The discharge tube is situated so that light oscillating between the resonator mirrors passes through these windows. In order to maximize transmission efficiency, frequently these windows are at Brewster's angle to the optical path. Since lasers tend to be relatively inefficient, they often are just above the threshold of operability. It is therefore very important that the optical losses in the cavity be kept to a minimum.
As a result of this requirement, the Brewster windows terminating the discharge tube, as well as other optical members whih may be inserted within the path of the optical cavity, are invariably made of fused silica, a form of vitreous silica. Fused silica is used for high quality optical applications because (1) it has very low optical losses, (2) it has a low thermal coefficient of expansion, and (3) it can be polished to a high quality finish.
Fused silica for high quality optical applications typically is made synthetically by vapor-phase hydrolysis. Oxygen is passed through a volatile silicon compound, such as silicon-tetrachloride and the resulting mixture is fed along with natural gas into a burner. Hydrogen is often used rather than natural gas. Hydrolysis takes place in the resulting flame, producing fused silica.
Fused silica should be distinguished from fused quartz, another vitreous silica, which is of lesser optical quality and is generally not used in applications such as lasers. Fused quartz is the material formed by direct melting of natural quartz crystals. For additional information about the properties of fused silica as well as other vitreous silica, such as fused quartz, reference is made to the Encyclopedia of Chemical Technology, Vol. 18, 2nd ed., John Wiley & Sons, Inc. 1969, pp. 73-105.
A perplexing problem which reduces laser efficiency is the phenomenon of a pink or reddish fluorescent light emission emanating from the Brewster windows terminating the laser discharge tube. This frequently observed phenomenon is observed in the section of the Brewster window through which the laser beam, oscillating between the resonator mirrors, passes. The fluorescence occurs from within the interior of the window and should be distinguished from the occasional surface fluorescent effects resulting from the deposition of miscellaneous materials on the surface of the window. The reasons for this fluorescence are not entirely understood but it is believed that it is a result of the bombardment of high energy visible and ultraviolet photons which are generated in the discharge tube plasma.
The amount of fluorescence can vary from window to window. It has also been observed that the amount of fluorescence usually increases with the usage of the laser. The presence of the red fluorescence is extremely detrimental to the laser performance. In effect, the energy which is used creating the fluorescent effect is energy which is taken out of the oscillating laser beam. Given the generally low efficiency of lasers, this means that the overall output energy can be seriously reduced by the fluorescent effect, or lasing action can be terminated altogether.
The problem is even more acute for wavelengths or lines having a lower intrinsic strength in the first place. This can be best explained by means of an example. The argon ion laser most frequently operates at 4880 A (blue) and 5145 A (green). However, an argon ion laser does have ultraviolet lines at 3638 A and 3511 A. Similarly, while krypton's most powerful wavelength is 6471 A (red), it has ultraviolet lines at 3507 A and 3564 A. As a general approximation, the visible lines of each of these two lasers are an order of magnitude greater in strength than the ultraviolet lines. As a result, if one operates or desires to operate an argon or krypton ion laser in the ultraviolet, one encounters a more serious deterioration, proportionately, in the output as a result of the fluorescence of the windows.
Another deleterious effect also occurs. It has been observed that the laser beam sometimes attempts to seek a lower loss path "around" the fluoresence. This causes a vibratory or dancing motion of the laser beam which can have the effect of altering the mode of the laser or, in some extreme situations, causing a total breakup of the beam itself.
Several approaches have been taken to solve this internal fluorescence problem. A common approach is simply to use fused silica windows which empirically have the lowest amount of fluorescence and then replace them after extended usage, since usage normally results in increasing fluorescence. Another approach has been to reduce the thickness of the windows which has the effect of reducing the overall intensity of the fluorescence. The former has the disadvantage that it is necessary to expend extensive amounts of time and labor to rebuild the laser and the latter has the problem that the thin windows can be distorted easily by mechanical strains thereby introducing optical inhomogeneities to the optical path. All of these approaches still assume that high quality glass must be used. Since these requirements are best met using fused silica, this has been the window material selected.